1. Field of the Invention
The separation of substantially pure metallic aluminum from minerals, oxides and impure metal by chemical methods which employ the combined actions of such reducing agents as carbon and silicon together with relatively nonvolatile, stable halides.
2. Description of the Prior Art
Direct reduction of aluminum from minerals or mixtures containing oxides or silicates of aluminum has been the subject of many investigations, publications and patents. The Cowles patents (e.g., U.S. Pat. No. 324,659) issued in the 1800's, taught a workable scheme, (practiced before the Hall-Heroult electrolysis became dominant) based on the reaction: EQU Al.sub.2 O.sub.3 +3C.fwdarw.2Al+3CO (1)
This reaction is very endothermic and, like all endothermic reactions, is favored by high temperature. Cowles used the electrical resistance of mixed solid reagents to develop temperatures in excess of 1800.degree. C. Many variations of what has come to be called "carbothermic" reduction were reduced to practice, but not necessarily commercialized. Difficulties included: (a) a tendency to form aluminum carbide; and (b) a tendency for back-reaction. The products of the reaction upon cooling can form the oxide, carbide and/or oxycarbide of aluminum, which are losses of metallic product aluminum. Cowles met these problems by incorporating copper or copper-oxide in the charge, thus producing an aluminum-copper alloy. In fact, many methods, including some quite recent, contemplate reduction to an alloy, followed by separate extraction of pur aluminum. For example (see Schmidt U.S. Pat. No. 3,254,988 and Wood U.S. Pat. No. 3,758,289) carbothermic reduction can lead to an alloy containing silicon and iron.
Silicon (in the form of ferrosilicon) has been proposed (see Schmidt U.S. Pat. No. 3,655,362) as a reducing agent to supplement or supplant carbon: EQU 2Al.sub.2 O.sub.3 +3Si.fwdarw.4Al+3SiO.sub.2 ( 2)
Advantages include: (a) aluminum and silicon here form no compounds; and (b) there is little tendency for back-reaction.
The main disadvantage of Reaction (2) is that it does not go to completion, and the aluminum-silicon alloy produced must be further processed if the objective is to secure pure aluminum.
Despite formidable technical obstacles, pursuit of direct reduction, ongoing for over a century, with yet no substantive commercial realization, continues to receive much attention, primarily because: (a) there is the compelling prospect of drastically reducing the energy consumed in producing aluminum; (b) low grade ores and byproducts may become viable sources of aluminum, without first extracting pure Al.sub.2 O.sub.3 ; and (c) direct reduction producing impure metal appears less of a deterrent with the possible emergence of halide refining.
Direct reduction can be coupled with halide separation in various ways: In general, the halogen elements (flourine, chlorine, bromine and iodine) find application in non-electrolytic thermochemical separation and refining, because (a) many halide compounds are volatile and so lend themselves to vapor phase separations of relatively non-volatile metals, and (b) some halide reactions, such as disproportionation condensation, are highly selective. The four halogens tend to behave similarly, but not identically. The fluorides and chlorides generally are more stable and have higher melting and boiling points than the bromides and iodides. Chlorine, the most abundant and least costly of the halogens, lends itself to aluminum separation and refining. However, in some schemes, fluorine or bromine offers specific advantages and cannot be ruled out. Cominations of halogens are always possible. But there are inevitable losses, and the overall view, from the standpoints both of economy and physical chemistry, is that the most likely reagents will be chlorides.
Pivotal to halide separation or refining is the disproportionation reaction: EQU 3AlCl.revreaction.AlCl.sub.3 +2Al (3)
Reaction (3), as written, is exothermic and takes place upon cooling, and the reverse reaction is endothermic, upon heating. The monohalide (or subhalide) AlCl gas is stable only above about 1200.degree. C., and "disproportionates" in accordance with Reaction (3) when cooled much below this temperature (assuming one atmosphere total pressure). AlCl.sub.3 (at lower temperatures, actually Al.sub.2 Cl.sub.6) is a gas, and Al a liquid, at the disproportionation temperature. Pure, solid AlCl.sub.3 sublimes at 180.degree. C., and is soluble in water and many molten salts. Reaction (3) probably was recognized in the middle 1940's. (See Gross U.S. Pat. No. 2,470,305) It is highly selective. The usual object in separation or refining is somehow to produce AlCl at high temperature, deposit pure liquid Al by disproportionation, and recycle the AlCl.sub.3.
Consider a simple hypothetical prior art arrangement for halide refining comprising a hot chamber, maintained well above 1200.degree. C., and a "cold" chamber well below 1200.degree. C. The chambers are connected by two gas conduits. Impure metallic aluminum is placed in the hot chamber; AlCl.sub.3 gas arrives through one conduit from the cold chamber; reverse Reaction (3) takes place in the hot chamber: AlCl gas passes through the other conduit to the cold chamber where it disproportionates into pure product aluminum and AlCl.sub.3 ; and AlCl.sub.3 is recycled back to the hot chamber. Impure aluminum would be continuously fed to, and residue (mostly iron and silicon) withdrawn from, the hot chamber; pure product aluminum would be withdrawn from the cold chamber. Theoretcally, at least, there would be no net consumption of chlorine; chlorine carrying aluminum would simply be recycled in a closed loop. (See Phillips U.S. Pat. Nos. 3,217,820 and 3,249,424; McGeer U.S. Pat. No. 3,243,282; Southam U.S. Pat. No. 3,292,914; and Dewing U.S. Pat. No. 3,436,210).
The great difficulty with halide refining has reposed with the handling of active, aggressive, corrosive, environmentally offensive halide vapors at high temperature. Halide refining has been seriously considered as a second step following simple direct reduction by carbon or silicon; the first step produces an alloy and the second, pure aluminum. A potential alternative is halide separation, i.e., combined reduction and halide treatment. It has long been perceived that chloridizing (or halidizing) reagents could react directly with impure aluminum oxide in the presence of carbon to produce AlCl gas, and, by disproportionation, pure aluminum (See Sparwald U.S. Pat. No. 3,186,832; Othmer U.S. Pat. Nos. 3,793,003; 3,853,541; and 3,856,508).
Many direct or indirect halidizing agents have been proposed, including: Cl.sub.2, HCl, FeCl.sub.2, SiCl.sub.4 and AlCl.sub.3 (Significantly, all of these are gases at temperatures well below that required for monohalide formation). Many chemical equations can be conceived, but a few suffice to represent the above thinking. For example, at relatively low temperature, elemental chlorine attacks oxides in concert with carbon: EQU Al.sub.2 O.sub.3 +3Cl.sub.2 +3C.fwdarw.2AlCl.sub.3 +3CO (4) EQU Fe.sub.2 O.sub.3 +2Cl.sub.2 +3C.fwdarw.2FeCl.sub.2 +3CO (5)
Intermediate chlorides further attack aluminum oxide: EQU Al.sub.2 O.sub.3 +3FeCl.sub.2 +3C.fwdarw.2AlCl.sub.3 +3Fe+3CO (6)
Aluminum trichloride reacts at higher temperature: EQU Al.sub.2 O.sub.3 +AlCl.sub.3 +3C.fwdarw.3AlCl+3CO (7)
Without exception, arrangements set forth in issued patents or published literature inherently embody manipulation of hot gaseous halides, a very serious drawback. This is only one consequence of incorporating a relatively high chemical potential of halogen in the system, a signal defect of published methods. High halogen activity also leads to losses of halogen through formation and discard of stable halides of alkali and alkaline earth metals (mainly CaCl.sub.2, MgCl.sub.2, NaCl and KCl); also, traces of other halide vapors (FeCl.sub.2 and SiCl.sub.4) contaminate the aluminum (with Fe and Si). Finally, severe environmental pollution is to be expected from any process involving high halogen activity (including Hall-Heroult electrolysis, the current mainstay of the industry.)
In addition, as with non-halide direct reduction, back-reactions present difficulties. When the disproportionation Reaction (3) takes place in the presence of CO, some aluminum reacts to form oxide and oxycarbide. Rapid cooling of the AlCl gas has been found necessary to minimize back-reactions. These and other problems have proven severe enough to prevent commercialization of direct halide reduction.
An interesting and important variant of halide reduction has enjoyed some success: alumina is reacted with carbon and elemental chlorine to produce AlCl.sub.3, which is then condensed, dissolved in a bath of molten salt (NaCl, LiCl and AlCl.sub.3), and electrolyzed to produce elemental Cl.sub.2 (thence recycled) and aluminum. At least one manifestation of the present invention takes advantage of and constitutes an improvement upon this developed technology.